1. Field of the Invention
The present invention relates to an electric power steering control apparatus that serves to assist a steering force of a driver by means of the rotational force of an electric motor (hereinafter also referred to simply as a “motor”).
2. Description of the Related Art
In general, an electric power steering apparatus is constructed such that the steering torque generated by driver's rotational operation of the steering wheel of a vehicle is detected, and a current corresponding to the steering torque is supplied to the electric motor, which is arranged to be in engagement with an appropriate mechanism of a steering system, to drive and rotate the electric motor, whereby assist torque to driver's steering torque is generated to perform a steering operation.
However, in such a steering system, there has been known that a tire vibration called shimmy generated due to the structure of the vehicle (i.e., generated in a rotational direction of the steering wheel during travel of the vehicle according to a deviation or shift of wheel balance) or a tire vibration called brake judder (i.e., generated upon brake operation due to the eccentricity of a brake rotor) is transmitted to the steering wheel, thereby generating a large vibration of the steering system.
FIG. 7 is an explanatory view that shows one example of the vibration of a steering system, wherein the relation between a shimmy vibration (steering torque) and the traveling speed of a vehicle (vehicle speed) is shown.
Thus, in order to suppress such the steering vibration, there has hitherto been proposed a technique that sets the handle shaft conversion inertia decided by the inertia of an electric motor (e.g., brushless motor) and the speed reduction gear ratio of a reduction gear mechanism to a value within the range from 4×10−2 [kg·m2] to 10×10−2 [kg·m2] (see, for example, a first patent document: Japanese patent application laid-open No. 2003-40120).
In this case, in order to suppress road noise such a kick back from a road surface, etc., a relatively large inertia of 4×10−2 [kg·m2] or more is permitted, and deterioration in steering feeling resulting from a sense of inertia due to the large inertia is supplemented by means of inertia compensation control of the steering system.
In addition, in the past, there has also been proposed a technique that sets a complementary sensitivity function with respect to vibration frequencies to a value near “1” in a frequency band where there exists an external disturbance which should not be suppressed, but sets to a value near “0” in a frequency band where there exists an external disturbance that should be transmitted (see, for instance, a second patent document: Japanese patent application laid-open No. 2001-334948).
In this case, too, external disturbances can be suppressed by permitting relatively large motor inertia. That is, with respect to unnecessary external disturbances (not to be transmitted), motor inertia is positively made use of, whereas motor inertia, which is felt by the driver when performing a steering operation, is compensated for in a torque control system.
In the conventional electric power steering control apparatuses as stated above, according to the techniques of the above-mentioned first and second patent documents, the vibration is suppressed by the large motor inertia, but the larger the inertia, the lower the resonance frequency of the steering mechanism becomes, so it is necessary to attenuate even the frequency band of load information in which there exist external disturbances that should be transmitted, thus giving rise to a problem that it becomes difficult to grasp road surface conditions.